The SNF1 protein kinase is required for expression of the invertase gene in response to glucose deprivation in Saccharomyces cerevisiae. We selected for genes that in multicopy suppress the invertase defect of temperature-sensitive snf1 mutants. Increased dosage of the MSN1 gene restores high-level, regulated invertase expression in snf1-ts mutants, and disruption of MSN1 in the wild type reduces invertase expression a fewfold. MSN1 gene dosage does not affect SNF1 protein kinase activity in vitro. MSN1 encodes a 43-kilodalton protein, and a MSN1-beta-galactosidase fusion protein was localized in the nucleus. A LexA-MSN1 fusion protein, when bound to a lexA operator, activates transcription of an adjacent promoter. In vitro synthesized MSN1 protein exhibits weak, nonspecific DNA-binding activity.